use HkdfExtract from x/crypto

[marten-seemann]

This means we don't have to expose it in qtls any more.

[codecov]

Codecov Report

Merging #3173 (22112b7) into master (31191f7) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3173      +/-   ##
==========================================
+ Coverage   85.43%   85.44%   +0.01%     
==========================================
  Files         133      133              
  Lines        9815     9813       -2     
==========================================
- Hits         8385     8384       -1     
+ Misses       1056     1055       -1     
  Partials      374      374              

Impacted Files	Coverage Δ
internal/qtls/go115.go	58.33% <ø> (+4.49%)	⬆️
internal/qtls/go116.go	58.33% <ø> (+4.49%)	⬆️
internal/handshake/initial_aead.go	100.00% <100.00%> (ø)
internal/utils/rand.go	62.50% <0.00%> (-12.50%)	⬇️

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